1. Field of the Invention
The present invention relates to an electrophotographic machine and more particularly to a method and apparatus for achieving registration of multiple color images in the process direction.
2. Description of the Related Art
An in-line color laser image printing process where the media passes through four color developing stations in series will print the same speed as monochrome. The photo conductive drum exposures must occur for all four colors at the proper time. Both process and scan direction alignment of the four color developing stations is critical. The process location of each scanning laser beam must overlap to prevent color offset in the process direction. If one beam is out of synchronization with the others, the image in the corresponding color will be misaligned relative to the remaining color components, and the resulting image blurred. Each color should have an adjustment to correct for process direction misalignment because each color has a scanning laser beam following a separate optical path. Thermal changes will cause misalignment so the adjustment should be performed regularly and not be just a one time manufacturing adjustment.
One technique for compensating for process direction errors involves the synthesis of a dot or PEL by controlling the power of the laser beam during the scanning of an adjacent pair of scan lines to form a synthesized dot which lies at a synthesized position between the adjacent pair of scan lines as disclosed in Hadady et al U.S. Pat. No. 6,229,555 assigned to the assignee of the present invention. In color printers, analog techniques for controlling the rotational phase of the polygonal mirrors, and the phase and period of rotational driving of the toner image carriers for each of several different monochrome color units to achieve synchronization among the units have been suggested. One system suggests a fine synchronizing control unit to control the rotational phase of the polygon mirrors by utilizing the outputs of three selectors which are input to the polygon mirror""s driving motors of the each of three monochromatic toner image forming units and the selection of the three selectors is varied in accordance with a position-shift correction quantity.
There remains a need for a simplistic yet effective technique for achieving good quality PEL registration among multiple printheads in the process direction.
The present invention provides process direction registration among several printheads by altering image translation time within the individual printheads.
The invention comprises, in one form thereof, coordinating the operation of a plurality of laser printheads which cooperate in the creation of an image by repetitive or periodic laser beam transverse scans to form images at respective first locations, translate the formed images to respective second locations, and transfer the translated images to a common image receiving medium, by starting the scanning of first and second printheads in a random phase relationship to one another and subsequently locking the scanning phase relationship between the first and second printheads. The locked scanning phase relationship is determined and the translation time of the image formed by the second printhead is altered by an amount determined, in part, by the determined phase relationship to achieve a predetermined apparent scanning phase relationship between the first and second printheads. Additional printheads may be similarly phased with the first printhead.
Also in general, and in one form of the invention, coordination of the operation of a plurality of laser printheads which cooperate in the creation of an image by periodic laser beam transverse scans to form images at respective first locations, translate the formed images to respective second locations, and transfer the translated images to a common image receiving medium is achieved by determining the longitudinal position of the transverse scan of each laser beam and then altering the translation time of the image formed by at least one printhead by an amount determined, in part, by a corresponding determined longitudinal position to achieve sub-pel registration. Alteration of the translation time of the image formed by additional ones of the printheads by amounts determined, in part, by the determined longitudinal position of the transverse scan of corresponding laser beams may occur.
An advantage of the present invention is that the coordination of images formed by several laser beams in a color printing environment is achieved.
Another advantage of the present invention is that the need for adjusting individual rotating polygonal mirrors to achieve proper phasing upon start-up is eliminated.
A further advantage of the present invention is that the same technique may be utilized to achieve proper phasing among multiple rotating mirrors and to correct for process direction laser beam variations.